Despite recent advances in understanding of molecular signaling involved in airway mucin gene expression, mechanisms of hyperplastic/metaplastic mucous transformation of bronchial epithelial cells are still poorly understood. Our previous studies have demonstrated that retinoic acid (RA) regulates phenotypical expression of human bronchial epithelial cells in culture. RA induced mucin gene expression and normal mucous cell differentiation, while IRA deficiency caused a squamous metaplasia. Interestingly, RA increased the activity of the MUCSAC promoter, which contains a cAMP response element-site. Recently, we found that the RA-deficient squamous metaplastic NHTBE cells contain little or no active cAMP-responsive element-binding protein (CREB). Surprisingly, we also found that RA activated CREB. We further found that the proinflammatory cytokine interleukin-1beta (IL-1beta) induced overexpression of the MUCSAC gene in the mucous NHTBE cells, but did not in the squamous metaplastic NHTBE cells, and at the same time activated CREB in the mucous, but not the squamous cells. On the basis of these results, we hypothesize CREB is a key molecule controlling differentiation of airway epithelial cells under normal and diseased conditions. Activation of CREB by RA is a prerequisite for basal MUC5AC expression and normal mucous differentiation, and chronic, persistent overexpression and activation of CREB by specific inflammatory mediators plays a critical role in mucous cell hyperplasia and/or metaplasia and resultant mucus hypersecretion. This application will address this hypothesis by determining the molecular mechanisms whereby CREB regulates normal and aberrant bronchial epithelial cell differentiation.